This invention concerns generally analysis of signals generated by a sensing device, and in particular concerns a method and apparatus for digitally resolving a pair of analog signals produced in phase quadrature by a conventional plunger-type gauge head responsive to movement of a plunger thereof.
Taking precise measurements of distances or displacements is often critical for quality control aspects of industrial processes. For example, parts often need to be manufactured to specific tolerances. Such parts could be processed along a conveyor belt, where they could be automatically measured for the accuracy (i.e. acceptability) of one or more of their critical dimensions. Those found to be out of tolerance could be automatically removed, or otherwise flagged for further attention. The need for precision can extend down to the single micrometmer level, and even fractions thereof. With the current general state of technology, measurements beyond 0.1 micrometers would generally not serve any practical purpose for some processes such as producing a work piece, but an accurate and repeatable measurement to such level can be useful. However, difficulty generally exists when working at such levels of precision, since almost any type or source of error in taking measurements or analyzing measurement signals will be highly significant.
Various devices exist for the purpose of making relatively high-precision measurements. One conventional device is a plunger-type gauge head, such as distributed by the Heidenhain Metro Company of Elk Grove Village, Ill. Generally, a plunger or shaft mechanism extends from a unitary gauge head and is fixed to an enclosed graticule having a predetermined grating pitch, such as about 10 micrometers. As the extended plunger or shaft is displaced by the distance to be measured, the graticule is also moved by precisely the same distance relative a light source on one side thereof and a pair of photocell elements on the other side thereof. Generally, each of the photocell elements produce a signal having an amplitude which varies in direct proportion with the amount of light received by such photocell element.
Without any plunger (i.e. graticule) movement, the photocell or light detecting elements produce a pair of DC signals, since there are no variations in the amount of light received. During graticule movement, such elements produce a pair of analog, generally sinusoidal, quadrature signals. The number of periods of such signals (which may for example be in the hundreds or more for a given plunger movement) is related to the distance moved by the graticule since the physical interruption of light by the grating thereon produces the sinusoidal signals. The quadrature relationship of such signals is due to the specific placement of the photocells from one another relative the grating pitch; as understood by those of ordinary skill in the art.
In general, each signal cycle indicates specific plunger movement, with the length of the cycle being related to the grating pitch of the graticule. If such pitch is 10 micrometers, for example, then each complete signal cycle (i.e. full period thereof) indicates 10 micrometers of plunger movement. Thus, the distance moved by the graticule (and hence the plunger) may be readily determined to a resolution of 10 micrometers by simply counting the number of whole signal cycles resulting from a given movement. The accuracy of such measurements is thus limited by the resolution of partial signal cycles for a final, uncompleted cycle of movement.
In general, one approach to such partial signal cycle resolution includes scaling the total movement value of a gauge signal cycle by the amount of partial movement within the final, uncompleted cycle. Since a full cycle is defined by 360 degrees of angular displacement of a gauge signal, partial cycle movement may be conveniently expressed as a number of degrees of angular displacement. A mathematical solution for such expression within an uncompleted signal cycle is provided by calculating the arc sin of the position amplitude divided by the maximum peak amplitude. In general, with respect to resolution accuracy, the critical value of such solution concerns measurement of the position amplitude.
One known solution for resolving analog, quadrature gauge signals from a typical gauge head (and particularly for determining the above-referenced critical position amplitude value) is the repeated segmentation (i.e. division) of such signals by electronic circuitry. However, in order to increase such resolution, it is necessary to increase the level or number of repetitions of such division. Each increase in division level requires virtually exponential increases in the number of additional circuit elements. As is well known, electronics are subject to inaccuracies caused by error and drift in the circuits due to changes in surrounding temperature, humidity, aging of the circuit elements and the like. The cumulative effects of such problems will obviously only be compounded by additional levels of circuitry. Furthermore, each additional level of circuitry adds to the cost of such resolving technique. Also, such technique compounds initial problems with the gauge signals themselves, such as any type of mismatch in the signal levels output by the pair of light detecting elements.
Another existing technique for resolving gauge signals to obtain the position amplitude and maximum peak amplitude values generally involves conversion of the analog gauge signals into digital signals, with subsequent processing thereof by digital electronics such as a microprocessor. U.S. Pat. No. 4,390,865 issued to Lauro is one example of a position measuring apparatus utilizing analog-to-digital conversion of measurement signals. While a digital processing approach favorably contributes to the accuracy of such technique, the required analog-to-digital conversion is generally a source of significant errors, which limit the accuracy and repeatability of relatively high-resolution measurements.
For example, as generally known in the art, analog-to-digital conversion typically involves successive approximations relative predetermined reference values. Such references are usually electronically produced, which means they are subject to fluctuation, just as are the analog circuits discussed above. Variation in such references from their intended predetermined values adversely affects the accuracy and repeatability of the analog-to-digital conversions. Even expensive methods of producing supposedly high-precision conversion references are subject to variation sufficient to be significant when attempting gauge signal resolution down to whole and fractional micrometers.
Furthermore, inaccuracies may occur when detecting signal zero-points or crossing-points for the purpose of determining the passage of whole and partial signal cycles. For example, basic signal amplitudes may relatively vary between the pair of gauge signals, such that crossing points thereof are distorted. Also, any differences in the basic DC levels of such pair of analog gauge signals may result in distortion of the crossing points. Either type of error tends to shift the distance value of whole and partial signal cycles from their nominal values, which accounts for a further possible source of error during attempted high-accuracy resolution of gauge signals.